Initial commit: Go SRE interview exercises

Includes solved 02-worker-pool and 09-safe-counter (mutex, atomic, sharded).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GbnJj9ZqEx9nUvzkjdwfMq

03-rate-limiter/main_test.go

@@ -0,0 +1,71 @@
+package main
+
+import (
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestBurstAllowedThenDenied(t *testing.T) {
+	l := NewLimiter(1, 3) // slow refill so it doesn't interfere
+	for i := 0; i < 3; i++ {
+		if !l.Allow() {
+			t.Fatalf("request %d within burst should be allowed", i)
+		}
+	}
+	if l.Allow() {
+		t.Fatal("4th immediate request should be denied (bucket empty)")
+	}
+}
+
+func TestRefillOverTime(t *testing.T) {
+	l := NewLimiter(20, 1) // 20 tokens/sec => 1 token per 50ms
+	if !l.Allow() {
+		t.Fatal("initial token missing")
+	}
+	if l.Allow() {
+		t.Fatal("bucket should be empty immediately after")
+	}
+	time.Sleep(120 * time.Millisecond) // ~2.4 tokens accrued, capped at 1
+	if !l.Allow() {
+		t.Fatal("token should have refilled after wait")
+	}
+}
+
+func TestBurstIsCapped(t *testing.T) {
+	l := NewLimiter(1000, 2)
+	time.Sleep(50 * time.Millisecond) // would accrue ~50 tokens uncapped
+	allowed := 0
+	for i := 0; i < 10; i++ {
+		if l.Allow() {
+			allowed++
+		}
+	}
+	if allowed > 2 {
+		t.Fatalf("bucket exceeded burst: %d immediate allows (cap 2) — idle time built infinite credit", allowed)
+	}
+}
+
+func TestConcurrentNoOverAdmission(t *testing.T) {
+	l := NewLimiter(0.0001, 100) // effectively no refill during the test
+	var allowed atomic.Int64
+	var wg sync.WaitGroup
+	for i := 0; i < 50; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for j := 0; j < 10; j++ {
+				if l.Allow() {
+					allowed.Add(1)
+				}
+			}
+		}()
+	}
+	wg.Wait()
+	if got := allowed.Load(); got > 100 {
+		t.Fatalf("over-admission under concurrency: %d > 100 tokens spent", got)
+	} else if got < 100 {
+		t.Fatalf("under-admission: only %d of 100 available tokens spent", got)
+	}
+}